Electronic device and method of driving the same

ABSTRACT

To provide a novel driving method for driving an electronic device by using digital gray scale and time gray scale in combination, which secures high duty ratio, which can display an image normally even when a sustain period is shorter than an address period, and which is hardly affected by dulled signal waveform. In a sub-frame period ( 102 ) where a sustain period is shorter than an address period, a clear period ( 105 ) is squeezed in between completion of a sustain period ( 104 ) and start of an address period of the subsequent sub-frame period. The length of the sustain period ( 104 ) thus can be set without being limited by the length of an address period ( 103 ). This non-display period is provided by changing the electric potential of a storage capacitor line. Therefore, unlike the case where the non-display period is provided by changing the electric potential of a cathode wiring, the present invention is hardly affected by dulled signal waveform.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of an electronic device.The invention particularly relates to a method of driving an activematrix electronic device having a thin film transistor (TFT) that isformed on an insulator and to an electronic device driven by the method.

2. Description of the Related Art

EL displays have lately been attracting attention as a flat paneldisplay to replace LCDs (liquid crystal displays), and researches arebeing actively made on the EL displays.

Similar to LCDs that have roughly two types of driving methods with onebeing passive matrix type used in, e.g., STN-LCDs and the other beingactive matrix type used in, e.g., TFT-LCDs, EL displays also are drivenby roughly two types of driving methods. One is passive matrix type andthe other is active matrix type.

In the case of the passive matrix type method, wirings to serve aselectrodes are arranged above and below an EL element. A voltage issequentially applied to the wirings to cause a current to flow in the ELelement, whereby the EL element emits light. In the active matrix typemethod, on the other hand, each pixel has a TFT so that signals can beheld in each pixel.

FIGS. 14A and 14B show an example of the structure of an active matrixelectronic device used in EL displays. FIG. 14A is a diagram showing theentire circuit structure in which a pixel portion 1453 is arranged inthe center of a substrate 1450. To the right and left of the pixelportion, gate signal line side driver circuits 1452 are arranged tocontrol gate signal lines. It is not necessary to place the gate signalline side driver circuits 1452 on both sides of the pixel portion butinstead one gate signal line side driver circuit may be provided on oneside thereof. However, considering circuit operation efficiency andreliability, it is desirable to arrange the driver circuits on bothsides. A source signal line side driver circuit 1451 for controllingsource signal lines is arranged above the pixel portion 1453. One ofpixels in FIG. 14A is enlarged and shown in FIG. 14B. Denoted by 1401 inFIG. 14B is a TFT functioning as a switching element when a signal iswritten in a pixel (hereinafter referred to as switching TFT). Referencesymbol 1402 denotes a TFT functioning as an element for controlling acurrent to be supplied to an EL element 1403 (current controllingelement) (the TFT will be referred to as EL driving TFT). According to ageneral and frequently employed method, a p-channel TFT is chosen as theEL driving TFT because source grounding is satisfactory in light of TFTbehavior and there are restrictions in manufacture of the EL drivingelement 1403, and the EL driving TFT 1402 is arranged between an anodeof the EL element 1403 and a current supply line 1407. Reference symbol1404 denotes a storage capacitor for holding a signal (voltage) inputtedfrom a source signal line 1406. The storage capacitor 1404 in FIG. 14Bhas one terminal connected to the current supply line 1407. However,wiring exclusive to the storage capacitor may be used instead. Theswitching TFT 1401 has a gate terminal connected to a gate signal line1405, and has a source terminal connected to the source signal line1406. The EL driving TFT 1402 has a drain terminal connected to an anodeor a cathode of the EL element 1403, and has a source terminal connectedto the current supply line 1407.

The EL element is comprised of an anode, a cathode, and a layercontaining an organic compound that provides electro luminescence(luminescence generated by applying electric field) (the layerhereinafter referred to as EL layer). The luminescence from an organiccompound can be divided into light emission upon returning from singletexcitation to the ground state (fluorescence) and light emission uponreturning from triplet excitation to the ground state (phosphorescence).Both kinds of light emission can be used in light emitting devices towhich the present invention is applicable.

The EL layer defined herein includes all the layers that are providedbetween an anode and a cathode. Specifically, the EL layer is comprisedof a light emitting layer, a hole injection layer, an electron injectionlayer, a hole transportation layer, an electron transportation layer,and some other layers. The basic structure of an EL element is alaminate in which an anode, a light emitting layer and a cathode aresequentially layered. Other types of EL layer structure are a laminatein which an anode, a hole injection layer, a light emitting layer and acathode are sequentially layered, and a laminate in which an anode, ahole injection layer, a light emitting layer, an electron transportationlayer and a cathode are sequentially layered.

The EL element in this specification refers to an element composed of ananode, an EL layer and a cathode.

Now, the circuit operation of the active matrix electronic device isdescribed with reference to FIGS. 14A and 14B. First, the gate signalline 1405 is selected to apply a voltage to a gate electrode of theswitching TFT 1401 and turn the switching TFT 1401 conductive. Thensignals (voltages) from the source signal line 1406 are accumulated inthe storage capacitor 1404. The voltage of the storage capacitor 1404serves as a gate-source voltage V_(GS) of the EL driving TFT 1402, andhence a current flows in the EL driving TFT 1402 and the EL element 1403in an amount corresponding to the voltage of the storage capacitor 1404.The EL element 1403 emits light as a result.

The luminance of the EL element 1403, namely, the amount of currentflowing through the EL element 1403 can be controlled by V_(GS) of theEL driving TFT 1402. V_(GS) is the voltage of the storage capacitor1404, which is equals to a signal (voltage) inputted to the sourcesignal line 1406. In short, the luminance of the EL element 1403 iscontrolled by controlling the signal (voltage) inputted to the sourcesignal line 1406. Lastly, the gate signal line 1405 is brought intonot-selected state to close the gate of the switching TFT 1401 and turnthe switching TFT 1401 unconductive. At this point, electric chargesaccumulated in the storage capacitor 1404 are held. Therefore, V_(GS) ofthe EL driving TFT 1402 is held as it is, and a current flows throughthe EL driving TFT 1402 into the EL element 1403 in an amountcorresponding to V_(GS).

Those described above have been reported in: SID 99 Digest, p. 372,“Current Status and Future of Light-emitting Polymer Display Driven byPoly-Si TFT”; ASIA DISPLAY 98, p. 217, “High Resolution Light EmittingPolymer Display Driven by Low Temperature Polysilicon Thin FilmTransistor with Integrated Driver”; Euro Display 99, Late News, p. 27,“3.8 Green OLED with Low Temperature Poly-Si TFT”; etc.

Gray scale display methods for EL displays can be divided into an analoggray scale method and a digital gray scale method. The former method,i.e., the analog gray scale method, changes the luminance in an analogfashion by changing the gate-source voltage V_(GS) of the EL driving TFT1402 to control the amount of current flowing into the EL element 1403.In contrast thereto, according to the latter method, i.e., the digitalgray scale method, there are only two states for the gate-source voltageV_(GS) of the EL driving TFT 1402. It is either that V_(GS) is in arange where the current is not at all allowed to flow in the EL element(less than “light-up start voltage”) or that V_(GS) is in a range wherethe maximum amount of current flows (equal to or larger than “luminancesaturation voltage”). Therefore the EL element is in either lights-onstate or lights-off state, and there is no other state.

The digital gray scale method is mainly used in EL displays, for imagedisplay through this method is hardly affected by fluctuation in TFTcharacteristics such as threshold value. However, the digital gray scalemethod by itself is only capable of displaying in two gray scales.Therefore, several techniques have been proposed to provide multi-grayscale display by combining the digital gray scale method with other grayscale methods.

One of those proposals is a combination of the digital gray scale methodand an area ratio gray scale method. The area ratio gray scale method isa method in which gray scale is obtained by controlling the area of thelit-up portions. To be specific, the method provides gray scale displayby dividing one pixel into a plurality of sub-pixels to control thenumber or the area of the lit-up sub-pixels. This method has a drawbackand there are difficulties in obtaining high resolution and multi-grayscale because the pixel can be divided into only a small number ofsub-pixels. This area ratio gray scale method is detailed in: EuroDisplay 99, Late News, p. 71, “TFT-LEPD with Image Uniformity by AreaRatio Grav Scale”; IEDM 99, p. 107, “Technology for Active Matrix LightEmitting Polymer Displays”; etc.

Another technique for obtaining multi-gray scale with the digital grayscale method is to combine the digital gray scale method with a timegray scale method. The time gray scale method obtains gray scale byutilizing the difference in length of the lights-on periods. To bespecific, gray scale is obtained in this method by dividing one frameperiod into a plurality of sub-frame periods to control the number orlength of the sub-frame periods during which EL elements emit light.

The digital gray scale method may be combined with the area ratio grayscale method and the time gray scale method, which is detailed in IDW99, p. 171, “Low-temperature Poly-Si TFT Driven Light-emitting-polymerDisplays and Digital Gray Scale for Uniformity”.

FIGS. 15A and 15B are timing charts in a driving method using thecombination of digital gray scale and time gray scale. An address(writing) period and a sustain (lights-on) period are completelyseparated from each other in a sub-frame period shown in FIG. 15A,whereas they are not separated in FIG. 15B.

In driving methods utilizing time gray scale, normally, an address(writing) period and a sustain (lights-on) period are needed for eachbit. According to a driving method where an address (writing) period anda sustain (lights-on) period are completely separated from each other (amethod where a sustain (lights-on) period in each sub-frame periodstarts only after an address (writing) period corresponding to onescreen writing is completed), the address (writing) periods take up alarge portion of one frame period. In addition, as shown in FIG. 15A,the address (writing) period has a period 1501 during which neitherwriting or lighting is carried out in rows other than a certain row aslong as the gate signal line of that certain row is selected. Thereforeduty ratio (the ratio of the length of sustain (Lights-on) periods inone frame) is very low. There is no other way than increasing theoperation clock to shorten the address (writing) periods and,considering margin for the operation of the circuit, only limited grayscale is possible. In contrast to this method, a driving method in whichan address (writing) period and a sustain (lights-on) period are notseparated from each other starts the sustain (lights-on) period for,e.g., the k-th row EL element immediately after the completion of thegate signal line selecting period for the k-th row gate signal line.Therefore some pixels are lit up during the gate signal line selectingperiods for the gate line signals of other rows, which makes thisdriving method advantageous in light of high duty ratio.

However, the method where an address (writing) period and a sustain(lights-on) period are not separated from each other has the followingproblem. The length of one address (writing) period extends from thestart of the gate signal line selecting period for the first row gatesignal line to the completion of the gate signal line selecting periodfor the last row gate signal line. Two different gate signal lines cannot simultaneously be selected at some points. Accordingly, in thedriving method where an address (writing) period and a sustain(lights-on) period are not separated from each other, the sustain(lights-on) period has to have a length equal to or longer than thelength of the address (writing) period (strictly speaking, a length of aperiod starting upon completion of writing a signal for the first rowgate signal line and ending with completion of writing a signal for thelast row gate signal line). Thus there is a limitation in setting theminimum unit for the sustain (lights-on) period when aiming atmulti-gray scale. The minimum unit in FIG. 15B corresponds to the lengthof a period denoted by 1502, where a period ending with completion of anaddress (writing) period Ta₄ of a minimum bit sub-frame period SF₄ doesnot overlap with a period starting upon start of the first address(writing) period of the next frame period. When a sustain (lights-on)period is shorter than the period 1502, normal display cannot beobtained. Since the length of a sustain (lights-on) period is determinedby the ratio of power of 2 in the combination of the digital gray scalemethod and the time gray scale method, obtaining multi-gray scale isdifficult within confinement set to the length of one frame period.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel driving methodmainly using digital gray scale and time gray scale in combination,which secures high duty ratio and which can display an image normallyeven when a sustain (lights-on) period is shorter than an address(writing) period.

In order to attain the above object, the present invention takes thefollowing measures.

A method of driving an electronic device according to the presentinvention makes it possible to set the length of a sustain (lights-on)period without being restricted by the length of an address (writing)period in a sub-frame period where a sustain (lights-on) period isshorter than an address (writing) period by squeezing a non-displayperiod for an EL element in between completion of the sustain(lights-on) period and start of an address (writing) period of the nextsub-frame period so that the address (writing) periods do not overlap.As a result, the overlap of the address (writing) periods can be avoidedand an image can be displayed normally even when a sustain (lights-on)period of less significant bit is shortened because of multi-gray scale.

The structure of an electronic device according to the present inventionwill be described below.

According to a first aspect of the present invention, there is provideda method of driving an electronic device, with one frame periodcomprising n sub-frame periods SF_(n), SF₂, . . . , SF_(n), the nsub-frame periods each comprising address (writing) periods Ta₁, Ta₂, .. . , Ta_(n) and sustain (lights-on) periods Ts₁, Ts₂, . . . , Ts_(n),characterized in that

-   -   the address (writing) period overlaps with the sustain        (lights-on) period in at least one sub-frame period of the n        sub-frame periods, and    -   that, in the case where an address (writing) period Ta_(m)        (1≦m≦n) of a sub-frame period SF_(m) overlaps with an address        (writing) period Ta_(m+1) of a sub-frame period SF_(m+1), a        clear period Tc_(m) is provided which starts upon completion of        a sustain (lights-on) period Ts_(m) of the sub-frame period        SF_(m) and ends upon start of the address (writing) period        Ta_(m+1).

According to a second aspect of the present invention, there is provideda method of driving an electronic device, with one frame periodcomprising n sub-frame periods SF₁, SF₂, . . . , SF_(n), the n sub-frameperiods each comprising address (writing) periods Ta₁, Ta₂, . . . ,Ta_(n) and sustain (lights-on) periods Ts₁, Ts₂, . . . , Ts_(n),characterized in that

-   -   the address (writing) period overlaps with the sustain        (lights-on) period in at least one sub-frame period of the n        sub-frame periods, and    -   that, in the case where an address (writing) period Ta_(n) of a        j-th (0<j) frame sub-frame period SF_(n) overlaps with an        address (writing) period Ta₁ of a (j+1)-th frame sub-frame        period SF₁, a clear period Tc_(n) is provided which starts upon        completion of a sustain (lights-on) period Ts_(n) of the j-th        frame sub-frame period SF_(n) and ends upon start of the address        (writing) period Ta₁ of the (j+1)-th frame sub-frame period SF₁.

According to a third aspect of the present invention, there is provideda method of driving an electronic device, with one frame periodcomprising n sub-frame periods SF₁, SF₂, . . . , SF_(n), the n sub-frameperiods each comprising address (writing) periods Ta₁, Ta₂, . . . ,Ta_(n) and sustain (lights-on) periods Ts₁, Ts₂, . . . , Ts_(n),characterized in that,

-   -   in a certain sub-frame period SF_(k)(1≦k≦n), when the length of        its address (writing) period is given as ta_(k) the length of        its sustain (lights-up) period as ts_(k) and the length of one        gate signal line selecting period as t_(g)(ta_(k), ts_(k),        t_(g)>0), and ta_(k)>ts_(k) is satisfied, the length of SF_(K)'s        clear period given as Tc_(k)(Tc_(k)>0) always satisfies the        following expression:        tc _(k) ≧ta _(k)−(tS _(k) +t _(g))

According to a fourth aspect of the present invention, a method ofdriving an electronic device of any one of the first to third aspects ofthe invention is characterized in that a clear signal inputted duringthe clear period is provided by increasing or lowering the electricpotential of a storage capacitor line by means of a signal inputted froma storage capacitor line driving circuit.

According to a fifth aspect of the present invention, a method ofdriving an electronic device of the fourth aspect of the invention ischaracterized in that an EL element does not emit light during the clearperiod irrespective of an image signal.

According to a sixth aspect of the present invention, there is providedan electronic device comprising a source signal line side drivercircuit, a gate signal line side driver circuit, a storage capacitorline driving circuit, and a pixel portion, characterized in that:

-   -   the pixel portion has a plurality of source signal lines, a        plurality of gate signal lines, a plurality of current supply        lines, a plurality of storage capacitor lines, and a plurality        of pixels;    -   each of the plurality of pixels has a switching transistor, an        EL driving transistor, a storage capacitor, and an EL element;    -   the switching transistor has a gate electrode electrically        connected to the gate signal line;    -   the switching transistor has a source region and a drain region        one of which is electrically connected to the source signal line        and the other of which is electrically connected to a gate        electrode of the EL driving transistor;    -   the storage capacitor has an electrode electrically connected to        the storage capacitor line and has another electrode        electrically connected to the gate electrode of the EL driving        transistor; and    -   the EL driving transistor has a source region and a drain region        one of which is electrically connected to the current supply        line and the other of which is electrically connected to one        electrode of the EL element.

According to a seventh aspect of the present invention, an electronicdevice of the sixth aspect of the invention is characterized in that thestorage capacitor line is electrically connected to the storagecapacitor line driving circuit so that a signal having amplitude isinputted to the storage capacitor line from the storage capacitor linedriving circuit.

According to an eighth aspect of the present invention, there isprovided an electronic device characterized in that it is operated by adriving method in which:

-   -   one frame period comprising n sub-frame periods SF₁, SF₂, . . .        , SF_(n);    -   the n sub-frame periods each comprising address (writing)        periods Ta₁, Ta₂, . . . , Ta_(n) and sustain (lights-on) periods        Ts₁, Ts₂, . . . , Ts;    -   the address (writing) period overlaps with the sustain        (lights-on) period in at least one sub-frame period of the n        sub-frame periods; and,    -   in the case where an address (writing) period Ta_(m)(1≦m≦n) of a        sub-frame period SF_(m) overlaps with an address (writing)        period Ta_(m+1) of a sub-frame period SF_(m+1), a clear period        Tc_(m) is provided which starts upon completion of a sustain        (lights-on) period Ts_(m) of the sub-frame period SF_(m) and        ends upon start of the address (writing) period Ta_(m+1).

According to a ninth aspect of the present invention, there is providedan electronic device characterized in that it is operated by a drivingmethod in which:

-   -   one frame period comprising n sub-frame periods SF₁, SF₂, . . .        , SF_(n);    -   the n sub-frame periods each comprising address (writing)        periods Ta₁, Ta₂, . . . , Ta_(n) and sustain (lights-on) periods        Ts₁, Ts₂, . . . , Ts_(n);    -   the address (writing) period overlaps with the sustain        (lights-on) period in at least one sub-frame period of the n        sub-frame periods; and,    -   in the case where an address (writing) period Ta_(n) of a j-th        (0<j) frame sub-frame period SF_(n) overlaps with an address        (writing) period Ta₁ of a (j+1)-th frame sub-frame period SF₁, a        clear period Tc_(n), is provided which starts upon completion of        a sustain (lights-on) period Ts_(n) of the j-th frame sub-frame        period SF_(n) and ends upon start of the address (writing)        period Ta₁ of the (j+1)-th frame sub-frame period SF₁.

According to a tenth aspect of the present invention, there is providedan electronic device characterized in that:

-   -   one frame period comprising n sub-frame periods SF₁, SF₂, . . .        , SF_(n);    -   the n sub-frame periods each comprising address (writing)        periods Ta₁, Ta₂, . . . , Ta_(n) and sustain (lights-on) periods        Ts₁, Ts₂, . . . , Ts_(n); and,    -   in a certain sub-frame period SF_(k)(1≦k≦n), when the length of        its address (writing) period is given as ta_(k), the length of        its sustain (lights-up) period as ts_(k), and the length of one        gate signal line selecting period as t_(g)(ta_(k), tS_(k),        t_(g)>0), and ta_(k)>ts_(k) is satisfied, the length of SF_(K)'s        clear period given as Tc_(k)(Tc_(k)>0) always satisfies the        following expression:        tc _(k) ≧ta _(k)−(ts _(k) +t _(g))

According to an eleventh aspect of the present invention, an electronicdevice of any one of the eighth to tenth aspects of the invention ischaracterized in that a clear signal inputted during the clear period isprovided by increasing or lowering the electric potential of a storagecapacitor line by means of a signal inputted from a storage capacitorline driving circuit.

According to a twelfth aspect of the present invention, an electronic ofthe eleventh aspect of the invention is characterized in that an ELelement does not emit light during the clear period irrespective of animage signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a timing chart illustrating a driving method according toEmbodiment 1 of the present invention;

FIGS. 2A and 2B are timing charts illustrating the driving methodaccording to Embodiment 1 of the present invention;

FIGS. 3A and 3B are timing charts illustrating a driving methodaccording to Embodiment 2 of the present invention;

FIGS. 4A to 4C are diagrams showing an exemplary process ofmanufacturing an electronic device in accordance with Embodiment 3;

FIGS. 5A to 5C are diagrams showing the exemplary process ofmanufacturing an electronic device in accordance with Embodiment 3;

FIGS. 6A and 6B are diagrams showing the exemplary process ofmanufacturing an electronic device in accordance with Embodiment 3:

FIGS. 7A and 7B show an electronic device according to Embodiment 4,where FIG. 7A is a top view thereof and FIG. 7B is a sectional viewthereof;

FIG. 8 is a sectional view showing a pixel portion of an electronicdevice according to Embodiment 5;

FIGS. 9A and 9B are diagrams showing an example of a process ofmanufacturing the electronic device according to Embodiment 5;

FIG. 10 is a sectional view showing a pixel portion of an electronicdevice according to Embodiment 6;

FIGS. 11A and 11B are diagrams showing an example of circuit structureof an electronic device according to Embodiment 7;

FIG. 12 is a timing chart illustrating a driving method according toEmbodiment 7 of the present invention;

FIG. 13 is a timing chart illustrating the driving method according toEmbodiment 7 of the present invention;

FIGS. 14A and 14B are diagrams showing an example of circuit structureof an electronic device;

FIGS. 15A and 15B are timing charts illustrating division of a frameperiod in time gray scale;

FIGS. 16A and 16B are diagrams showing an example of circuit structureof an electronic device;

FIGS. 17A and 17B are diagrams showing an example of circuit structureof an electronic device;

FIGS. 18A and 18B are diagrams illustrating the electric potential ofsignals in the respective portions in a driving method of the presentinvention;

FIGS. 19A and 19B are diagrams illustrating the electric potential ofsignals in the respective portions in a driving method of the presentinvention;

FIGS. 20A and 20B are diagrams showing an example of circuit structureof an electronic device according to Embodiment 1;

FIGS. 21A and 21B are diagrams showing an example of circuit structureof an electronic device according to Embodiment 8;

FIGS. 22A to 22F are diagrams showing examples of an electronic machineof Embodiment 10, to which a method of driving an electronic deviceaccording to the present invention is applied; and

FIGS. 23A and 23B are diagrams showing examples of the electronicmachine of Embodiment 10, to which a method of driving an electronicdevice according to the present invention is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure of the present invention will be described.

A normal structure of a pixel portion is as shown in FIGS. 16A and 16Bin which one terminal of a storage capacitor 1604 is connected to acurrent supply line 1607. The current supply line usually has itselectric potential kept constant. The pixel portion may alternatively bestructured as shown in FIGS. 17A and 17B in which a storage capacitorline 1711 is provided and one terminal of a storage capacitor 1704 isconnected to the storage capacitor line. In this case, the electricpotential of the storage capacitor line 1711 is kept constant.

The present invention adopts the circuit structure of FIGS. 17A and 17B,and no particular addition is required. However, this circuit structureis modified in the present invention such that the electric potential ofthe storage capacitor line 1711 is not always constant and a signal canbe inputted using a circuit.

The electric potential of the storage capacitor line 1711 is keptconstant during an address (writing) period and a sustain (lights-on)period. When a non-display period is provided irrespective of a gatevoltage of an EL driving TFT 1702, the electric potential of the storagecapacitor line 1711 is increased. (This applies to the case where the ELdriving TFT 1702 is a p-channel TFT. If an n-channel TFT is usedinstead, the reverse operation takes place.) This will be called a clearsignal and a period during which the clear signal is inputted will becalled a clear period. Accompanying with this increase in electricpotential, a gate-source voltage V_(GS) of the EL driving TFT 1702electrically connected to the storage capacitor 1704 is also increasedto force it into OFF state. Supply of current to an EL element 1703 isthus stopped in this period irrespective of a signal being written,thereby providing a clear period.

When the electric potential of the storage capacitor line 1711 is keptconstant during an address (writing) period and a sustain (lights-on)period, somewhat low electric potential is preferable. The electricpotential of the storage capacitor line has to be increased uponinputting a clear signal to a value higher than in the period A, i.e.,the period during which the electric potential of the storage capacitorline 1711 is kept constant. If the electric potential in the period A ishigh from the start, it has to be increased even higher in inputting aclear signal. Therefore the electric potential is preferably low in theperiod A. (This applies to the case where the EL driving TFT 1702 is ap-channel TFT. Since the reverse operation takes place when an n-channelTFT is used instead, the electric potential is preferably kept high inthe period A in this case.)

According to the driving method of the present invention, a clear signalis inputted to the storage capacitor line 1711 to squeeze in a clearperiod. Therefore a sustain (lights-on) period shorter than an address(writing) period can easily be set by changing the length of the clearperiod. This and the high duty ratio described above together work veryfavorably in obtaining multi-gray scale.

In order to prevent the EL element 1703 from emitting light whateversignal is inputted from the signal line, for instance, the difference inelectric potential between an anode 1709 of the EL element and a cathode1710 thereof is set to zero. Another example is to cut off supply ofcurrent to the EL element 1703 by providing a current cutting TFTbetween the EL driving TFT 1702 and the EL element 1703 and turning thecurrent cutting TFT unconductive. However, in these methods, the periodsmay not be in timing with each other if the waveform of a signalinputted is dulled (referring to a phenomenon in which a signal isdelayed or dulled upon rising or falling of a pulse). This problembecomes more prominent as the length of the respective periods is setshorter. Moreover, the aperture ratio of the pixel may be lowereddepending on the type of the current cutting TFT. In contrast thereto,the driving method of the present invention prevents the EL element fromemitting light by changing the electric potential of the storagecapacitor line to release the electric charges from the storagecapacitor. Therefore it is not necessary to control the electricpotential of a signal line related to an image (video) signal for thenon-display period. The dulled signal waveform thus does not affect thepresent invention, nor the current cutting TFT that may lower theaperture ratio is necessary.

Next, the electric potential pattern of the respective portions will bedescribed with reference to FIGS. 18A and 18B. The circuit discussedhere is of FIGS. 17A and 17B again.

In FIGS. 18A and 18B, reference symbol 1801 denotes the electricpotential of the source signal line 1706, 1802 denotes the electricpotential of a gate electrode of the EL driving TFT 1702, 1803 denotesthe electric potential of the gate signal line 1705, and 1804 denotesthe electric potential of the storage capacitor line 1711. Shown inFIGS. 18A and 18B is the case in which the polarity of the switching TFT1701 is of n-channel whereas the polarity of the EL driving TFT 1702 isof p-channel. First, The electric potential 1804 of the storagecapacitor line 1711 is kept to a certain value. The electric potentialthereof is desirably kept low, for it has to be increased later. Signalsare then inputted to the source signal line 1706 and the gate signalline 1705, and writing into pixels is started.

FIG. 18A shows the case in which a LO signal is inputted to the gateelectrode of the EL driving TFT 1702 whereas FIG. 18B shows the case inwhich a Hi signal is inputted to the gate electrode of the EL drivingTFT 1702. In FIG. 18A, as the gate signal line 1705 is selected, a LOsignal is inputted to the gate electrode of the EL driving TFT 1702 tolower the electric potential thereof and turn it conductive so that theEL element 1703 starts to emit light. On the other hand, in FIG. 18B, aHi signal is inputted to the gate electrode of the EL driving TFT 1702to turn it unconductive as the gate signal line 1705 is selected.Therefore the EL element 1703 does not emit light. Then the period forselecting the gate signal line 1705 comes to an end, lowering theelectric potential of the gate signal line 1705. However, the electricpotential applied to the gate electrode of the EL driving TFT 1702 iskept, owing to the storage capacitor 1704, to the same value as theperiod during which the gate signal line 1705 is selected, so that theEL element 1703 continues to emit light in the case of FIG. 18A andlights-off state is continued in the case of FIG. 18B.

Described next is behavior of the respective portions before and after aclear period in the present invention. The electric potential 1804 ofthe storage capacitor line 1711 is increased at points indicated by thedotted line X-X′ in FIGS. 18A and 18B. It is desirable here to set theamplitude of the electric potential 1804 of the storage capacitor line1711 larger than the amplitude of the electric potential of the sourcesignal line 1706. At this point, the period for selecting the gatesignal line 1705 has ended and the switching TFT 1701 has already beenturned unconductive. The voltage between the two terminals of thestorage capacitor 1704 is stored as it is, and an increase in electricpotential 1804 of the storage capacitor line 1711 connected to oneterminal of the storage capacitor causes an increase in electricpotential of the other terminal thereof, namely, an increase in gatevoltage 1802 of the EL driving TFT 1702. Therefore the electricpotential 1802 of the gate electrode of the EL driving TFT 1702 is risenat the point indicated by the dotted line X-X′ in FIG. 18A. This makesthe EL driving TFT 1702 unconductive, thereby stopping supply of currentto the EL element 1703 and bringing it into lights-off state. Similarly,in FIG. 18B, an increase in electric potential 1804 of the storagecapacitor line 1711 is accompanied with an increase in electricpotential 1802 of the gate electrode of the EL driving TFT 1702.However, no change is caused in this case and the non-display statecontinues.

Through such an operation, the EL element 1703 can be forced intonon-display state even when the gate signal line 1705 is selected and asignal from the source signal line 1706 is being written in pixels inanother row. Accordingly, the length of a sustain (lights-on) period canbe set to any arbitrary value by changing the length of the clearperiod.

Although an n-channel TFT is used for the switching TFT 1701 in thecases shown in FIGS. 18A and 18B, the driving method of the presentinvention can work normally without any problem also when a p-channelTFT is used for the switching TFT. This case will be described belowwith reference to FIGS. 19A and 19B. The circuit discussed here is ofFIGS. 17A and 17B once again.

First, an electric potential 1904 of the storage capacitor line 1711 iskept constant. The electric potential 1904 is kept low from the reasondescribe above. Then signals are inputted to the source signal line 1706and the gate signal line 1705 and writing into pixels is started.

FIG. 19A shows the case in which a LO signal is inputted to the gateelectrode of the EL driving TFT 1702 whereas FIG. 19B shows the case inwhich a Hi signal is inputted to the gate electrode of the EL drivingTFT 1702. In FIG. 19A, as the gate signal line 1705 is selected, a LOsignal is inputted to the gate electrode of the EL driving TFT 1702 tolower the electric potential thereof and turn it conductive so that theEL element 1703 starts to emit light. On the other hand, in FIG. 19B, aHi signal is inputted to the gate electrode of the EL driving TFT 1702to turn it unconductive as the gate signal line 1705 is selected.Therefore the EL element 1703 does not emit light. Then the period forselecting the gate signal line 1705 comes to an end, lowering theelectric potential of the gate signal line 1705. However, the electricpotential applied to the gate electrode of the EL driving TFT 1702 iskept, owing to the storage capacitor, to the same value as the periodduring which the gate signal line 1705 is selected, so that the ELelement 1703 continues to emit light in the case of FIG. 19A andlights-off state is prolonged in the case of FIG. 19B.

Described next is behavior of the respective portions before and after aclear period in the present invention. The electric potential 1904 ofthe storage capacitor line 1711 is increased at points indicated by thedotted line Y-Y′ in FIGS. 19A and 19B. At this point, the period forselecting the gate signal line 1705 has ended and the switching TFT 1701has already been turned unconductive. The voltage between the twoterminals of the storage capacitor 1704 is stored as it is, and anincrease in electric potential 1904 of the storage capacitor line 1711connected to one terminal of the storage capacitor causes a simultaneousincrease in gate voltage 1902 of the EL driving TFT 1702. Thus theelectric potential 1902 of the gate electrode of the EL driving TFT 1702is increased at the point indicated by the dotted line Y-Y′ in FIG. 19A.This makes the EL driving TFT 1702 unconductive, thereby stopping supplyof current to the EL element 1703 and bringing it into lights-off state.In FIG. 19B, an increase in electric potential of the storage capacitorline 1711 causes a simultaneous increase in electric potential 1902 ofthe gate electrode of the EL driving TFT 1702. This also causes anincrease in electric potential on the source side of the switching TFT1701. Since the polarity of the switching TFT 1701 here is of p-channel,the switching TFT 1701 is temporarily turned conductive when the sourceside electric potential is increased. This brings a change towardequalization of the source-drain electric potential of the switching TFT1701. In other words, the electric potential 1902 of the gate electrodeof the EL driving TFT 1702 is lowered. An electric potential 1903 of thegate signal line 1705 is constant at this point, and hence a decrease inelectric potential 1902 of the gate electrode of the EL driving TFT 1702is accompanied with a decrease in electric potential on the source sideof the switching TFT 1701. This brings a change toward reduction ingate-source voltage of the switching TFT 1702. When the gate-sourcevoltage reaches lower than the threshold voltage of the switching TFT1701, the switching TFT 1701 is returned to unconductive state. Therespective portions behave as above when a p-channel TFT is used for theswitching TFT 1701. Whichever polarity the switching TFT 1701 takes, theEL driving TFT 1702 is turned unconductive when the electric potentialof the storage capacitor line 1711 is increased.

As has been described, the driving method of the present invention canwork normally irrespective of whether the polarity of the switching TFT1701 is of n-channel or p-channel.

The present invention has been described in this embodiment mode takingas an example the case where the time gray scale method and the digitalgray scale method are used in combination. If the area ratio gray scalemethod is added thereto, it is still possible to bring the EL elementinto non-display state through the same manner.

Now, descriptions are given on embodiments of the present invention.

Embodiment 1

FIG. 20A shows an example of the entire circuit structure. A pixelportion is placed at the center. FIG. 20B is a circuit diagram of onepixel enclosed in a dotted line frame 2000. A source signal line sidedriver circuit is arranged above the pixel portion. A gate signal lineside driver circuit is put to the left of the pixel portion. A storagecapacitor line driving circuit is set to the right of the pixel portion.

The actual driving method will be described using a timing chart.Discussed here is a driving method that uses digital gray scale and timegray scale in combination to obtain n bit gray scale display. Forsimplification, n is set to 3 here and display of 2³=8 gray scales willbe described. The circuit diagrams of FIGS. 20A and 20B are referred toagain.

FIG. 1 is a timing chart of the electric potential of the gate signalline and of the storage capacitor line in the respective rows in thiscase. According to the circuit used in this embodiment, a switching TFT2001 is an n-channel TFT. Therefore, in a gate signal line selectingperiod, the electric potential of a gate signal line 2005 is increasedand the switching TFT 2001 is turned conductive.

The descriptions are given in temporal order. First, one frame periodhas to be divided into n sub-frames in order to obtain n bit gray scaledisplay. In this embodiment, n bit is 3 bit and one frame is dividedinto 3 sub-frame periods SF₁, to SF₃. Each sub-frame period has address(writing) periods Ta₁ to Ta₃ and sustain (lights-on) periods Ts₁, toTs₃. An address (writing) period is a period required for writing of onescreen, and hence each address (writing) period has the same length asanother address (writing) period. The length of the sustain (lights-on)periods is determined in accordance with power of 2. Specifically, Ts₁:Ts₂: Ts₃=4: 2: 1 in the case of FIG. 1.

However, the length of the sustain (lights-on) period may not alwaysfollow the power of 2 to obtain gray scale display.

In the timing chart according to this embodiment, an address (writing)period and a sustain (lights-on) period are not completely separatedfrom each other and one of the sustain (lights-on) periods is shorterthan an address (writing) period. The gate signal line 2005 is firstselected one by one in SF₁, during which signals are written intopixels. When writing into pixels of one row is completed (when the gatesignal line selecting period is ended), this row enters the sustain(lights-on) period Ts₁.

Upon ending of the sustain (lights-on) period Ts₁, in SF₁, SF₂ isstarted and, as in SF₁, the gate signal line 2005 is selected one by oneto write signals into pixels. During the signals are written, theelectric potential of a storage capacitor line 2011 is kept constant.

Thereafter SF₃ is started. In SF₃, as shown in FIG. 1. the sustain(lights-on) period Ts₃ is shorter than the address (writing) period Ta₃.Then if the sustain (lights-on) period is started after completion ofthe address (writing) period and the next sub-frame period is startedimmediately after the sustain (lights-on) period is ended as in theforegoing sub-frame periods, the address (writing) period Ta₁ in SF₁ ofthe next frame period is started before completion of the address(writing) period Ta₃ in SF₃ as shown in FIG. 2A. Thus address (writing)periods of different sub-frames partially overlap with each other. Theexistence of this overlap period means that there are gate signal linesof two different rows which are simultaneously selected, and threatensnormal image display.

Therefore a period during which an EL element 2003 does not emit lightis squeezed in between the end of a certain period (sustain (lights-on)period) from the completion of Ts₃ and start of the next address(writing) period by increasing the electric potential of the storagecapacitor line 2011. This period for clearing the EL element 2003 isreferred to as clear period Tc_(n) (n corresponds to the sub-framenumber). In FIG. 2B, Tc₃ is provided after the end of Ts₃ so thatoverlap of Ta₃ with the next Ta₁ can be avoided and normal image displaycan be ensured.

The clear period is limited in length and it always has to satisfytc_(k)≧ta_(k)−(ts_(k)+t_(g)), where ta_(k) is the length of an address(writing) period Ta_(k) of a sub-frame period SF_(k)(1≦k≦n), Ta_(k)being shorter than a sustain (lights-on) period Ts_(k) thereof, ts_(k)is the length of the sustain (lights-on) period Ts_(k), t_(g) is thelength of one gate signal line selecting period (ta_(k), ts_(k),t_(g)>0), and tc_(k)(tc_(k)>0) is the length of a clear period inSF_(k).

Embodiment 2

Described in Embodiment 2 is a case in which the number of gray scalesis larger than in Embodiment 1, and there are plural sustain (lights-on)periods each of which is shorter than an address (writing) period. Thecircuit here is the same as the one in Embodiment 1 and reference isagain made to FIGS. 20A and 20B.

In this embodiment, display of 5 bit (2⁵=32) gray scales is discussed.Similar to the case of 3 bit gray scale display, address (writing)periods Ta₁ to Ta₅ all have the same length and sustain (lights-on)periods Ts₁ to Ts₅ are set so as to satisfy Ts₁: Ts₂: Ts₃: Ts₄: Ts₅=16:8: 4: 2: 1. Out of all the sustain (lights-on) periods, Ts₃, Ts₄ and Ts₅are each shorter than an address (writing) period.

In a driving method in which the EL element 2003 starts to emit lightimmediately after writing of signals is completed, if the next address(writing) period is started after the end of a sustain (lights-on)period, address (writing) periods of different sub-frame periodspartially overlap with each other as shown in FIG. 3A. Ta3 and Ta4overlap with each other in a range denoted by a in FIG. 3A, Ta4 and Ta5overlap with each other in a range denoted by b, and Ta4 and Ta5 and Ta1of the next sub-frame period (Ta1′) overlap in a range denoted by c. Thelength of the minimum unit sustain (lights-on) period becomes shorter asthe number of gray scales is increased, and three or more address(writing) periods may overlap as above. Therefore, similar to Embodiment1, clear periods Tc3, Tc4 and Tc5 are provided as shown in FIG. 3B inthe respective periods between the end of the sustain (lights-on) periodand start of the next address (writing) period. The overlap of address(writing) periods thus can be avoided and normal image display can beensured.

Embodiment 3

A method of manufacturing TFTs of a driver circuit (n-channel type TFTor p-channel type TFT) provided in the pixel portion and the peripheryof a pixel portion on the same substrate is explained in detail in thisembodiment.

First, as shown in FIG. 4A, a base film 5002 made of an insulating filmsuch as a silicon oxide film, a silicon nitride film, or a siliconoxynitride film is formed on a substrate 5001 made from glass, such asbarium borosilicate glass or aluminum borosilicate glass, typicallyCorning Corp. #7059 glass or #1737 glass. For example, a siliconoxynitride film 5002 a manufactured from SiH₄, NH₃, and N₂O by plasmaCVD is formed with a thickness of 10 to 200 nm (preferably from 50 to100 nm), and a hydrogenized silicon oxynitride film 5002 b with athickness of 50 to 200 nm (preferably between 100 and 150 nm),manufactured from SiH₄ and N₂O, is similarly formed and laminated. Thebase film 5002 with the two layer structure is shown in Embodiment 3,but the base film 5002 may also be formed as a single layer of one ofthe above insulating films, and it may be formed having a laminationstructure in which two or more layers are laminated.

Island shape semiconductor layers 5003 to 5006 are formed of crystallinesemiconductor film manufactured by using a laser crystalline method or aknown thermal crystallization method with a semiconductor film having anamorphous structure. The thickness of the island shape semiconductorlayers 5003 to 5006 is set from 25 to 80 nm (preferably between 30 and60 nm). There are no limitations on the crystalline semiconductor filmmaterial, but it is preferable to form the film from a semiconductormaterial such as silicon or a silicon germanium (SiGe) alloy.

A laser such as a pulse oscillation type or continuous emission typeexcimer laser, a YAG laser, or a YVO₄ laser can be used as a laser lightsource in manufacturing the crystalline semiconductor film with thelaser crystallization method. A method of condensing laser light emittedfrom a laser oscillator into a linear shape by an optical system andthen irradiating the light to the semiconductor film may be employedwhen these types of lasers are used. The crystallization conditions maybe suitably selected by the operator. However, the pulse oscillationfrequency is set to 30 Hz, and the laser energy density is set form 100to 400 mJ/cm² (typically between 200 and 300 mJ/cm²) when using theexcimer laser. Further, the second harmonic is utilized when using theYAG laser, the pulse oscillation frequency is set from 1 to 10 KHz, andthe laser energy density may be set from 300 to 600 mJ/cm² (typicallybetween 350 and 500 mJ/cm²). The laser light which has been condensedinto a linear shape with a width of 100 to 1000 μm, for example 400 μm,is then irradiated onto the entire surface of the substrate. This isperformed with an overlap ratio of 80 to 98% for the linear laser light.

A gate insulating film 5007 is formed covering the island shapesemiconductor layers 5003 to 5006. The gate insulating film 5007 isformed of an insulating film containing silicon a thickness of 40 to 150nm by plasma CVD or sputtering. A 120 nm thick silicon oxynitride filmis formed in Embodiment 3. The gate insulating film is not limited tothis type of silicon oxynitride film, of course, and other insulatingfilms containing silicon may also be used, in a single layer or in alamination structure. For example, when using a silicon oxide film, itcan be formed by plasma CVD with a mixture of TEOS (tetraethylorthosilicate) and O₂, at a reaction pressure of 40 Pa, with thesubstrate temperature set from 300 to 400° C., and by discharging at ahigh frequency (13.56 MHz) electric power density of 0.5 to 0.8 W/cm².Good characteristics as a gate insulating film can be obtained bysubsequently performing thermal annealing, at between 400 and 500° C.,of the silicon oxide film thus manufactured.

A first conductive film 5008 and a second conductive film 5009 are thenformed on the gate insulating film 5007 in order to form gateelectrodes. The first conductive film 5008 is formed from Ta with athickness of 50 to 100 nm, and the second conductive film 5009 is formedby W with a thickness of 100 to 300 nm, in Embodiment 3.

The Ta film is formed by sputtering, and sputtering with a Ta target isperformed by using Ar. If appropriate amounts of Xe and Kr are added tothe Ar during sputtering, the internal stress of the Ta film will berelaxed, and film peeling can be prevented. The resistivity of a phaseTa film is on the order of 20 μΩcm, and it can be used in the gateelectrode, but the resistivity of β phase Ta film is on the order of 180μΩcm and it is unsuitable for the gate electrode. The α phase Ta filmcan easily be obtained if a tantalum nitride film, which possesses acrystal structure near that of a phase Ta, is formed with a thickness of10 to 50 nm as a base for Ta in order to form the phase Ta film.

A W film is formed by sputtering with a W target. The W film can also beformed by thermal CVD using tungsten hexafluoride (WF₆). Whichever isused, it is necessary to make the film become low resistance in order touse it as the gate electrode, and it is preferable that the resistivityof the W film be made equal to or less than 20 μΩcm. The resistivity canbe lowered by enlarging the crystals of the W film, but for cases inwhich there are many impurity elements such as oxygen within the W film,crystallization is inhibited, and the film becomes high resistance. A Wtarget having a purity of 99.9999% is thus used in sputtering. Inaddition, the W film is formed while sufficient care is taken in orderthat no impurities from within the gas phase are introduced at the timeof film formation. Thus, a resistivity of 9 to 20 μΩcm can be achieved.

Note that, although the first conductive film 5008 is Ta and the secondconductive film 5009 is W in Embodiment 3, the conductive films are notlimited to these. Both the first conductive film 5008 and the secondconductive film 5009 may also be formed from an element selected fromthe group consisting of Ta, W, Ti, Mo, Al, and Cu, from an alloymaterial having one of these elements as its main constituent, or from achemical compound of these elements. Further, a semiconductor film,typically a polysilicon film, into which an impurity element such asphosphorous is doped may also be used. Examples of preferablecombinations other than that used in Embodiment 3 include: a combinationof the first conductive film formed from tantalum nitride (TaN) and thesecond conductive film formed from W; a combination of the firstconductive film formed from tantalum nitride (TaN) and the secondconductive film formed from Al; and a combination of the firstconductive film formed from tantalum nitride (TaN) and the secondconductive film formed from Cu.

A mask 5010 is formed next from resist, and a first etching process isperformed in order to form electrodes and wirings. An ICP (inductivelycoupled plasma) etching method is used in Embodiment 3. A gas mixture ofCF₄ and Cl₂ is used as an etching gas, and a plasma is generated byapplying a 500 W RF electric power (13.56 MHz) to a coil shape electrodeat 1 Pa. A 100 W RF electric power (13.56 MHz) is also applied to thesubstrate side (test piece stage), effectively applying a negativeself-bias. The W film and the Ta film are both etched on the same orderwhen CF₄ and Cl₂ are combined.

Edge portions of the first conducting layer and the second conductinglayer are made into a tapered shape in accordance with the effect of thebias voltage applied to the substrate side with the above etchingconditions by using a suitable resist mask shape. The angle of thetapered portions is from 15 to 45. The etching time may be increased byapproximately 10 to 20% in order to perform etching without any residueremaining on the gate insulating film. The selectivity of a siliconoxynitride film with respect to a W film is from 2 to 4 (typically 3),and therefore approximately 20 to 50 nm of the exposed surface of thesilicon oxynitride film is etched by this over-etching process. Firstshape conductive layers 5011 to 5016 (first conductive layers 5011 a to5016 a and second conductive layers 5011 b to 5016 b) composed of thefirst conducting layer and the second conducting layer are thus formedby the first etching process. Portions of the gate insulating film 5007not covered by the first shape conductive layers 5011 to 5016 are etchedon the order of 20 to 50 nm, forming thinner regions. (See FIG. 4A.)

A first doping process is then performed, and an impurity element whichimparts n-type conductivity is added. (See FIG. 4B.) Ion doping or ioninjection may be performed as the doping method. Ion doping is performedat conditions in which the dosage is set to 1×10¹³ to 5×10¹⁴ atoms/cm²,and an acceleration voltage is set between 60 and 100 keV. An elementresiding in group 15 of the periodic table, typically phosphorous (P) orarsenic (As), is used as the n-type conductivity imparting impurityelement. Phosphorous (P) is used here. The conductive layers 5011 to5015 become masks with respect to the n-type conductivity impartingimpurity element, and first impurity regions 5017 to 5025 are formed ina self-aligning manner. The impurity element which imparts n-typeconductivity is added to the first impurity regions 5017 to 5025 at aconcentration within a range of 1×10²⁰ and 1×10²¹ atoms/cm³.

A second etching process is performed next as shown in FIG. 4C. The ICPetching method is similarly used; a mixture of CF₄, Cl₂, and O₂ is usedas the etching gas, and a plasma is generated by supplying a 500 W RFelectric power (13.56 MHz) to a coil shape electrode at a pressure of 1Pa. A 50 W RF electric power (13.56 MHz) is applied to the substrateside (test piece stage), and a self-bias voltage which is lower incomparison with the first etching process is applied. The W film isetched anisotropically under these etching conditions, and Ta (the firstconductive layers) is anisotropically etched at a slower etching speed,forming second shape conductive layers 5026 to 5031 (first conductivelayers 5026 a to 5031 a and second conductive layers 5026 b to 5031 b).The gate insulating film 5007 is additionally etched on the order of 20to 50 nm, forming thinner regions, in regions not covered by the secondshape conductive layers 5026 to 5031.

The etching reaction of the W film or the Ta film in accordance with themixed gas of CF₄ and Cl₂ can be estimated from the generated radicals,or from the ion types and vapor pressures of the reaction products.Comparing the vapor pressures of W and Ta fluorides and chlorides, the Wfluoride compound WF₆ is extremely high, and the vapor pressures ofWCl₅, TaF₅, and TaCl₅ are of similar order. Therefore the W film and theTa film are both etched by the CF₄ and Cl₂ gas mixture. However, if asuitable quantity of O₂ is added to this gas mixture, CF₄ and O₂ react,forming CO and F, and a large amount of F radicals or F ions aregenerated. As a result, the etching speed of the W film having a highfluoride vapor pressure becomes high. On the other hand, even if Fincreases, the etching speed of Ta does not relatively increase.Further, Ta is easily oxidized compared to W, and therefore the surfaceof Ta is oxidized by the addition of O₂. The etching speed of the Tafilm is further reduced because Ta oxides do not react with fluorine andchlorine. It therefore becomes possible to have a difference in etchingspeeds of the W film and the Ta film, and it becomes possible to makethe etching speed of the W film larger than that of the Ta film.

A second doping process is then performed as shown in FIG. 5A. In thiscase, an impurity element which imparts n-type conductivity is dopedunder conditions of a lower dosage than that in the first dopingprocess, and at a higher acceleration voltage than that in the firstdoping process. For example, doping may be performed at an accelerationvoltage of 70 to 120 keV and with a dosage of 1×10¹³ atoms/cm², formingnew impurity regions inside the first impurity regions formed in theisland shape semiconductor layers of FIG. 11B. Doping is performed withthe second shape conductive layers 5026 to 5030 as masks with respect tothe impurity element, and doping is done such that the impurity elementis also added to regions below the second conductive layers 5026 a to5030 a. Third impurity regions 5032 to 5041, which overlap with thesecond conductive layers 5026 a to 5030 a, and second impurity regions5042 to 5051 between the first impurity regions and the third impurityregions are thus formed. The n-type conductivity imparting impurityelement is added at a concentration of 1×10¹⁷ to 1×10¹⁹ atoms/cm³ in thesecond impurity regions, and at a concentration of 1×10¹⁶ to 1×10¹⁸atoms/cm³ in the third impurity regions.

Then, as shown in FIG. 5B, fourth impurity regions 5052 to 5063, havinga conductivity type opposite to the single conductivity type, are formedin the island shape semiconductor layers 5004 and 5006, which formp-channel TFTs. The second conductive layers 5027 b and 5030 b are usedas masks against the impurity element, and the impurity regions areformed in a self-aligning manner. The island shape semiconductor layer5003 and 5005 which forms an n-channel TFT, and the second conductinglayer 5031 which forms a wiring, have their entire surfaces covered by aresist mask 5200 at this point. Phosphorous is added at differingconcentrations to the impurity regions 5052 to 5063, respectively, byion doping using diborane (B₂H₆). The impurity concentration in all ofthe regions is set so as to be from 2×10²⁰ to 2×10²¹ atoms/cm³.

The impurity regions are formed in the respective island shapesemiconductor layers by the above processes. The second shape conductivelayers 5026 to 5030, which overlap with the island shape semiconductorlayers, function as gate electrodes. Further, the numeral 5031 functionsas a source signal line.

A process of activating the impurity elements added to the respectiveisland shape semiconductor layers is then performed as shown in FIG. 5C,with the aim of controlling conductivity type. Thermal annealing usingan annealing furnace is performed for this process. In addition, laserannealing and rapid thermal annealing (RTA) can also be applied. Thermalannealing is performed with an oxygen concentration equal to or lessthan 1 ppm, preferably equal to or less than 0.1 ppm, in a nitrogenenvironment at 400 to 700° C., typically between 500 and 600 C. Heattreatment is performed for 4 hours at 500° C. in Embodiment 3. However,for cases in which the wiring material used in the second shape layers5026 to 5031 is weak with respect to heat, it is preferable to performactivation after forming an interlayer insulating film (having siliconas its main constituent) in order to protect the wirings.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation of the island shape semiconductor layers. Thisprocess is one of terminating dangling bonds in the island shapesemiconductor layers by hydrogen which is thermally excited. Plasmahydrogenation (using hydrogen excited by a plasma) may also be performedas another means of hydrogenation.

As shown in FIG. 6A, the fiirst interlayer insulating film 5064 isformed to 100 to 200 nm thick from the silicon oxynitride film. Afterthe second interlayer insulating film 5065 made of organic insulatingmaterials is formed, contact holes are formed in the first interlayerinsulating film 5064, the second interlayer insulating film 5065, andthe gate insulting film 5007. After forming each wiring (includingconnection electrodes) 5066 to 5071 and 5073, by patterning, a pixelelectrode 5072 contacting the connection electrode 5071 is formed bypatterning.

An organic resin material is used for the second interlayer insulatingfilm 5065. Organic resins such as polyimide, polyamide, acrylic, and BCB(benzocyclobutene) can be used. In particular, it is preferable to useacrylic, which has superior levelness for the second interlayerinsulating film 5065, because it is formed with a strong implication ofleveling. An acrylic film is formed in Embodiment 3 at a film thicknessat which steps formed by the TFTs can be sufficiently leveled. The filmthickness is preferably from 1 to 5 μm (more preferably between 2 and 4μm).

Formation of the contact holes is done using dry etching or wet etching,and contact holes for reaching the n-type impurity regions 5017 to 5021and 5023 to 5025 or the p-type impurity regions 5052 to 5063, a contacthole for reaching the wiring 5031, a contact hole for reaching theelectric current supply line (not shown in the figure), and contactholes for reaching the gate electrodes (not shown in the figure) areformed.

Further, a three layer structure lamination film, in which a 100 nmthick Ti film, a 300 nm thick Al film containing Ti, and a 150 nm thickTi film are formed in succession by sputtering and then patterned into apredetermined shape, is used for the wirings (a connecting wiring and asignal wiring are included) 5066 to 5071 and 5073. Of course, otherconductive films may be used.

An indium oxide tin oxide (ITO) film is formed as the pixel electrode5072 with a thickness of 110 nm in Embodiment 3, and patterning is thenperformed. The contact is attained such that the pixel electrode 5072 isarranged so as to contact and overlap with the connection electrode5071. Further, a transparent conductive film in which between 2 and 20%zinc oxide (ZnO) is mixed with indium oxide may also be used. The pixelelectrode 5072 becomes an anode of an EL element. (See FIG. 6A)

Next, as shown in FIG. 6B, an insulating film containing silicon (asilicon oxide film in Embodiment 3) is formed with a thickness of 500nm, an open portion is formed in a location corresponding to the pixelelectrode 5072, and a third interlayer insulating film 5074 is formed.Sidewalls can easily be formed into a tapered shape by using wet etchingwhen forming the open portion. If the sidewalls of the open portion arenot sufficiently gentle, then there is a conspicuous problem in whichthe EL layers degrade due to the step.

An EL layer 5075 and a cathode (MgAg electrode) 5076 are formed next insuccession, without exposure to the atmosphere, using vacuumevaporation. Note that the film thickness of the EL layer 5075 may beset from 80 to 200 nm (typically between 100 and 120 nm), and thethickness of the cathode 5076 may be set from 180 to 300 nm (typically200 to 250 nm).

The EL layer and the cathode are formed one after another with respectto pixels corresponding to the color red, pixels corresponding to thecolor green, and pixels corresponding to the color blue. However, the ELlayer is weak with respect to a solution, and therefore each of thecolors must be formed separately without using a photolithographytechnique. It is preferable to cover areas outside the desired pixelsusing a metal mask, and selectively form the EL layer and the cathodeonly in the necessary locations.

Namely, a mask is first set so as to cover all pixels other than onescorresponding to the color red, and a red color light emitting EL layerand a cathode are selectively formed using the mask. Next, a mask is setso as to cover all pixels other than ones corresponding to the colorgreen, and a green color light emitting EL layer and a cathode areselectively formed. A mask is similarly set so as to cover all pixelsother than ones corresponding to the color blue, and a blue color lightemitting EL layer and a cathode are selectively formed. Note thatalthough it is stated here that all differing masks are used, the samemask may also be reused. Further, it is preferable to perform processingup through the formation of the EL layers and the cathodes in all pixelswithout breaking the vacuum.

A method of forming three types of EL elements corresponding to eachcolor RGB is used here, but methods such as a method of combining colorfilters and white color light emitting EL elements; a method ofcombining blue color or blue-green color light emitting EL elements anda fluorescing body (fluorescing color conversion layer, CCM), and amethod of utilizing transparent electrodes in the cathodes (opposingelectrodes) and overlapping EL elements corresponding to RGB may also beused.

Note that known materials may be used for the EL layer 5075. Organicmaterials may be preferably used as the known materials, taking a drivervoltage into consideration. For example, a four layer structure of ahole injecting layer, a hole transporting layer, a light emitting layer,and an electron injecting layer may be used. Further, an example ofusing an MgAg electrode as the cathode of the EL element is shown inEmbodiment 3, but it is also possible to use other known materials.

A protecting electrode 5077 is formed next, covering the EL layers andthe cathodes. A conductive film having aluminum as its main constituentmay be used as the protecting electrode 5077. The protecting electrode5077 may be formed by vacuum evaporation using a different mask fromthat used in forming the EL layers and the cathodes. Further, it ispreferable to form the protecting electrode after forming the EL layersand the cathodes, without exposure to the atmosphere.

Finally, a passivation film 5078 is formed of a silicon nitride filmwith a thickness of 300 nm. In practice, the protecting electrode 5088fulfills a role of protecting the EL layers from contaminants such asmoisture, and in addition, the reliability of the EL elements can beadditionally increased by forming the passivation film 5078.

An active matrix electronic device having a structure like that shown inFIG. 6B is thus completed. Note that, in the manufacturing processes forthe active matrix electronic device in Embodiment 3, the source signallines are formed by Ta and W, materials used to form the gateelectrodes, due to the circuit structure and processing considerations.In addition, the gate signal lines are formed by Al, the wiring materialused in forming the source and drain electrodes. However, differentmaterials may also be used.

In the active matrix substrate of Embodiment 3 an extremely highreliability is thus shown, and the operating characteristics are alsoimproved not only in the pixel portion, but also in the driver circuitportion by arranging TFTs having suitable structures. It is alsopossible to add a metallic catalyst such as Ni in the crystallizationstep, thereby increasing crystallinity. It therefore becomes possible toset the driving frequency of the source signal line driver circuit to 10MHz or higher.

First, a TFT having a structure in which hot carrier injection isreduced without even a small drop in the operating speed is used as ann-channel TFT of a CMOS circuit forming the driver circuit portion. Notethat the driver circuit referred to here includes circuits such as ashift register, a buffer, a level shifter, a latch in line-sequentialdrive, and a transmission gate in dot-sequential drive.

In Embodiment 3, the active layer of the n-channel TFT contains a sourceregion, a drain region, a GOLD region, an LDD region, and a channelforming region, and the GOLD region overlaps with the gate electrodethrough the gate insulating film.

Further, there is not much need to worry about degradation due to hotcarrier injection with the p-channel TFT of the CMOS circuit, andtherefore LDD regions need not be formed in particular. It is of coursepossible to form an LDD region similar to that of the n-channel TFT, asa measure against hot carriers.

In addition, when using a CMOS circuit in which electric current flowsin both directions in the channel forming region, namely a CMOS circuitin which the roles of the source region and the drain regioninterchange, it is preferable that LDD regions be formed on both sidesof the channel forming region of the n-channel TFT forming the CMOScircuit, sandwiching the channel forming region. A circuit such as atransmission gate used in dot-sequential drive can be given as such anexample. Further, when a CMOS circuit in which it is necessary tosuppress the value of the off current as much as possible is used, then-channel TFT forming the CMOS circuit preferably has a structure inwhich a portion of the LDD region overlaps with the gate electrodethrough the gate insulating film. A circuit such as the transmissiongate used in dot-sequential drive can be given as such an example.

Note that, in practice, it is preferable to perform packaging (sealing),without exposure to the atmosphere, using a protective film (such as alaminated film or an ultraviolet hardened resin film) having goodairtight characteristics and little outgassing, and a transparentsealing material, after completing through the state of FIG. 6B. At thistime, the reliability of the EL element is increased by making an inertatmosphere on the inside of the sealing material and by arranging adrying agent (barium oxide, for example) inside the sealing material.

Furthermore, after the airtight properties have been increased inaccordance with the packaging process, a connector (flexible printedcircuit, FPC) is attached in order to connect terminals drawn from theelements and circuits formed on the substrate with external signalterminals. And a finished product is completed. The device in this stateat which it is ready for delivery as the product is referred to as anelectronic device throughout this specification.

Furthermore, in accordance with the processes shown in Embodiment 3, theactive matrix substrate can be manufactured by using five photomasks (anisland shape semiconductor layer pattern, a first wiring pattern (gatewiting, island-like source wiring, capacitor wirings), an n-channelregion mask pattern, a contact hole pattern, and a second wiring pattern(including pixel electrodes and connection electrodes). As a result, theprocesses can be reduced, and this contributes to a reduction in themanufacturing costs and an increase in throughput.

Embodiment 4

An example of manufacturing an electronic device using the presentinvention is explained in embodiment 4

FIG. 7A is a top view of an electronic device using the presentinvention. FIG. 7B is a cross sectional view taken along the line X-X′of FIG. 7A. In FIG. 7A, reference numeral 4001 is a substrate, referencenumeral 4002 is a pixel portion, reference numeral 4003 is a sourcesignal side driver circuit, and reference numeral 4004 is a gate signalside driver circuit. The driver circuits are connected to externalequipment, through an FPC 4008, via wirings 4005, 4006 and 4007.

A covering material 4009, an airtight sealing material 4010 and asealing material (also referred to as a housing material shown in FIG.7B) 4011 are formed so as to enclose at least the pixel portion,preferably the driver circuits and the pixel portion, at this point.

Further, FIG. 7B is a cross sectional structure of the electronic deviceof the present invention. A driver circuit TFT 4013 (note that a CMOScircuit in which an n-channel TFT and a p-channel TFT are combined isshown in the figure here), a pixel portion TFT 4014 (note that only anEL driver TFT for controlling the current flowing to an EL element isshown here) are formed on a base film 4012 on a substrate 4001. The TFrsmay be formed using a known structure (a top gate structure or a bottomgate structure).

After the driver circuit TFT 4013 and the pixel portion TFT 4014 arecompleted, a pixel electrode 4016 is formed on an interlayer insulatingfilm (leveling film) 4015 made from a resin material. The pixelelectrode is formed from a transparent conducting film for electricallyconnecting to a drain of the pixel TFT 4014. An indium oxide and tinoxide compound (referred to as ITO) or an indium oxide and zinc oxidecompound can be used as the transparent conducting film. An insulatingfilm 4017 is formed after forming the pixel electrode 4016, and an openportion is formed on the pixel electrode 4016.

An EL layer 4018 is formed next. The EL layer 4018 may be formed havinga lamination structure, or a single layer structure, by freely combiningknown EL materials (such as a hole injecting layer, a hole transportinglayer, a light emitting layer, an electron transporting layer, and anelectron injecting layer). A known technique may be used to determinewhich structure to use. Further, EL materials exist as low molecularweight materials and high molecular weight (polymer) materials.Evaporation is used when using a low molecular weight material, but itis possible to use easy methods such as spin coating, printing, and inkjet printing when a high molecular weight material is employed.

In embodiment 4, the EL layer is formed by evaporation using a shadowmask. Color display becomes possible by forming emitting layers (a redcolor emitting layer, a green color emitting layer, and a blue coloremitting layer), capable of emitting light having different wavelengths,for each pixel using a shadow mask. In addition, methods such as amethod of combining a charge coupled layer (CCM) and color filters, anda method of combining a white color light emitting layer and colorfilters may also be used. Of course, the electronic device can also bemade to emit a single color of light.

After forming the EL layer 4018, a cathode 4019 is formed on the ELlayer. It is preferable to remove as much as possible any moisture oroxygen existing in the interface between the cathode 4019 and the ELlayer 4018. It is therefore necessary to use a method of depositing theEL layer 4018 and the cathode 4019 in an inert gas atmosphere or withina vacuum. The above film deposition becomes possible in embodiment 4 byusing a multi-chamber method (cluster tool method) film depositionapparatus.

Note that a lamination structure of a LiF (lithium fluoride) film and anAl (aluminum) film is used in embodiment 4 as the cathode 4019.Specifically, a 1 nm thick LiF (lithium fluoride) film is formed byevaporation on the EL layer 4018, and a 300 nm thick aluminum film isformed on the LiF film. An MgAg electrode, a known cathode material, mayof course also be used. The wiring 4007 is then connected to the cathode4019 in a region denoted by reference numeral 4020. The wiring 4007 isan electric power supply line for imparting a predetermined voltage tothe cathode 4019, and is connected to the FPC 4008 through a conductingpaste material 4021.

In order to electrically connect the cathode 4019 and the wiring 4007 inthe region denoted by reference numeral 4020, it is necessary to form acontact hole in the interlayer insulating film 4015 and the insulatingfilm 4017. The contact holes may be formed at the time of etching theinterlayer insulating film 4015 (when forming a contact hole for thepixel electrode) and at the time of etching the insulating film 4017(when forming the opening portion before forming the EL layer). Further,when etching the insulating film 4017, etching may be performed all theway to the interlayer insulating film 4015 at one time. A good contacthole can be formed in this case, provided that the interlayer insulatingfilm 4015 and the insulating film 4017 are the same resin material.

A passivation film 4022, a filling material 4023, and the coveringmaterial 4009 are formed covering the surface of the EL element thusmade.

In addition, the sealing material 4011 is formed between the coveringmaterial 4009 and the substrate 4001, so as to surround the EL elementportion, and the airtight sealing material (the second sealing material)4010 is formed on the outside of the sealing material 4011.

The filling material 4023 functions as an adhesive for bonding thecovering material 4009 at this point. PVC (polyvinyl chloride), epoxyresin, silicone resin, PVB (polyvinyl butyral), and EVA (ethylene vinylacetate) can be used as the filling material 4023. If a drying agent isformed on the inside of the filling material 4023, then it can continueto maintain a moisture absorbing effect, which is preferable. Further,deterioration of the EL layer may be suppressed by arranging a materialsuch as an oxidation preventing agent having an oxygen capturing effectinside the filler material 4023.

Further, spacers may be contained within the filling material 4023. Thespacers may be a powdered substance such as BaO, giving the spacersthemselves the ability to absorb moisture.

When using spacers, the passivation film 4022 can relieve the spacerpressure. Further, a film such as a resin film can be formed separatelyfrom the passivation film 4022 to relieve the spacer pressure.

Furthermore, a glass plate, an aluminum plate, a stainless steel plate,an FRP (fiberglass-reinforced plastic) plate, a PVF (polyvinyl fluoride)film, a Mylar film, a polyester film, and an acrylic film can be used asthe covering material 4009. Note that if PVB or EVA is used as thefilling material 4023, it is preferable to use a sheet with a structurein which several tens of μm aluminum foil is sandwiched by a PVF film ora Mylar film.

However, depending upon the light emission direction from the EL element(the light radiation direction), it is necessary for the coveringmaterial 4009 to have light transmitting characteristics.

Further, the wiring 4007 is electrically connected to the FPC 4008through a gap between the sealing material 4011 and the substrate 4001.Note that although an explanation of the wiring 4007 has been made here,the wirings 4005 and 4006 are also electrically connected to the FPC4008 by similarly passing underneath the sealing material 4011 andsealing material 4010.

In this embodiment, the covering material 4009 is bonded after formingthe filling material 4023, and the sealing material 4011 is attached soas to cover the lateral surfaces (exposed surfaces) of the fillingmaterial 4023, but the filling material 4023 may also be formed afterattaching the covering material 4009 and the sealing material 4011. Inthis case, a filling material injection opening is formed through a gapformed by the substrate 4001, the covering material 4009, and thesealing material 4011. The gap is set into a vacuum state (a pressureequal to or less than 10⁻² Torr), and after immersing the injectionopening in the tank holding the filling material, the air pressureoutside of the gap is made higher than the air pressure within the gap,and the filling material fills the gap.

Embodiment 5

A more detailed cross sectional structure of a pixel portion of theelectronic device is shown here in FIG. 8.

A switching TFT 4502 formed on a substrate 4501 is manufactured by usinga n-channel type TFT. A double gate structure is used in embodiment 4.In this embodiment, although a double gate structure is used, sincethere is no big difference in the structure and fabricating process,explanation is omitted. However, a structure in which two TFTs aresubstantially connected in series with each other is obtained byadopting the double gate structure, and there is a merit that an offcurrent value can be decreased. Further although the double gatestructure is used in this embodiment, a single gate structure, a triplegate structure, and a multi gate structure possessing a greater numberof gates may also be used.

Further, an EL driving TFT 4503 is formed by using an n-channel TFT. Adrain wiring 4504 of the switching TFT 4502 is electrically connected toa gate electrode 4506 of the EL driving TFT 4503 through a wiring (notshown in figure).

In a case where a driving voltage of the electronic device is high (10Vor more), a driver circuit TFT, in particular an N-channel type TFT, hashigh fear of deterioration due to hot carriers or the like. Thus, it isvery effective to adopt a structure in which an LDD region (GOLD region)is provided at a drain side of the N-channel type TFT, or at source anddrain sides so as to overlap with a gate electrode through a gateinsulating film, as shown in FIG. 6B of Embodiment 3. In a case where adriving voltage is low (10V or less), there is no fear of deteriorationdue to hot carrier. Thus, as shown in FIG. 8 of Embodiment 6, there isno need to provide a GOLD region. However, with respect to the switchingTFT 4502 in a pixel portion, it is very effective to adopt a structurein which an LDD region is provided at a drain side of the N-channel typeTFT, or at source and drain sides so as not to overlap with a gateelectrode through a gate insulating film to reduce an off-current. Atthis time, with respect to the EL driving TFT 4503, there is no need toprovide an LDD region, however, a private (dedicated) mask is necessaryto cover the portion of the EL driving TFT 4503 with a resist when anLDD region is formed in the switching TFT 4502. Therefore, in Embodiment6, the EL driving TFT 4503 is formed with the same structure (thestructure having an LDD region) as that of the switching TFT 4502 toreduce the mask number.

The manufacturing processes of TFTs having a structure shown inEmbodiment 6 will be described herein with reference to FIG. 9.

FIG. 9A shows a state which is obtained after the processes illustratedin FIG. 4B are completed in accordance with Embodiment 3. By employingup to the processes, first impurity regions 4701 to 4705 are formed.Subsequently, a first conductive film made of a Ta film and a secondconductive film made of a W film are etched as shown in FIG. 9B, andsecond impurity regions 4706 to 4711 having lower concentration thanthat of the first impurity regions are formed inside the first impurityregions formed in an island-like semiconductor layer in FIG. 9A. Thusformed second impurity regions 4706 to 4711 will be the above mentionedLDD region.

In accordance with Embodiment 3, again, an active matrix substrate maybe completed by employing processes shown after FIG. 5B.

In this embodiment, although the EL driving TFT 4503 is shown as asingle gate structure, a multi-gate structure in which a plurality ofTFTs are connected in series with each other may be adopted. Further,such a structure may be adopted that a plurality of TFTs are connectedin parallel with each other to substantially divide a channel formingregion into plural portions, so that radiation of heat can be made athigh efficiency. Such structure is effective as a countermeasure againstdeterioration due to heat.

Further, the wiring (not shown in figure) including the gate electrode4506 of the EL driving TFT 4503 partly overlaps with a drain wiring 4512of the EL driving TFT 4503 through an insulating film, and a storagecapacitor is formed in the region. The storage capacitor functions tostore a voltage applied to the gate electrode 4506 of the EL driving TFT4503.

A first interlayer insulating film 4514 is provided on the switching TFT4502 and the EL driving TFT 4503, and a second interlayer insulatingfilm 4515 made of a resin insulating film is formed thereon.

Furthermore, reference numeral 4517 denotes a pixel electrode (ELelement cathode) made from a conducting film with high reflectivity, andthis is electrically connected to a drain region of the EL driver TFT4503. It is preferable to use a low resistance conducting film, such asan aluminum alloy film, a copper alloy film, and a silver alloy film, ora laminate of such films. Of course, a lamination structure with anotherconducting film may also be used.

Next, the organic resin film 4516 is formed in the pixel electrode 4517and the EL layer 4519 is formed after patterning is performed on thefacing portion of the pixel electrode 4517. Although not shown in thefigures here, but the light emitting layer may be divided to correspondto each of the colors R (red), G (green), and B (blue). A conjugatepolymer material is used as an organic EL material. Polyparaphenylenevinylenes (PPVs), polyvinyl carbazoles (PVKs), and polyfluoranes can begiven as typical polymer materials.

Note that there are several types of PPV organic EL materials, andmaterials recorded in Schenk, H., Becker, H., Gelsen, O., Kluge, E.,Kreuder, W., and Spreitzer, H., Polymers for Light Emitting Diodes, EuroDisplay Proceedings, 1999, pp. 33-7, and in Japanese Patent ApplicationLaid-open No. Hei 10-92576, for example, may be used.

As specific light emitting layers, cyano-polyphenylene vinylene may beused as a red light radiating luminescence layer, polyphenylene vinylenemay be used as a green light radiating luminescence layer, andpolyphenylene vinylene or polyalkylphenylene may be used as a blue lightradiating luminescence layer. The film thicknesses may be between 30 and150 nm (preferably between 40 and 100 nm).

However, the above example is one example of the organic EL materialswhich can be used as luminescence layers, and it is not necessary tolimit use to these materials. An EL layer (a layer for emitting lightand for performing carrier motion for such) may be formed by freelycombining light emitting layers, electric charge transporting layers,and electric charge injecting layers.

For example, Embodiment 5 shows an example of using a polymer materialas a light emitting layer, but a low molecular weight organic ELmaterial may also be used. Further, it is possible to use inorganicmaterials such as silicon carbide, as an electric charge transportinglayer or an electric charge injecting layer. Known materials can be usedfor these organic EL materials and inorganic materials.

An EL element 4510 is complete at the point where the anode 4523 isformed. Note that what is called the EL element 4510 indicates the pixelelectrode (cathode) 4517 and the retention capacitor formed by the lightemitting layer 4519, hole injecting layer 4522 and the anode 4523.

In addition, a passivation film 4524 is then formed on the anode 4523 inEmbodiment 5. It is preferable to use a silicon nitride film or anoxidized silicon nitride film as the passivation film 4524. The purposeof this is the isolation of the EL element from the outside, and this ismeaningful in preventing degradation due to oxidation of the organic ELmaterial, and in controlling gaseous emitted from the organic ELmaterial. The reliability of the electronic device can thus be raised.

The EL display panel of Embodiment 5 has a pixel portion made frompixels structured as in FIG. 8, and has a switching TFT with asufficiently low off current value, and a EL driving TFT which is strongwith respect to hot carrier injection. An electronic device having highreliability, and in which good image display is possible, can thereforebe obtained.

In the case that the EL element having the structure explained in thisembodiment, the light emitted from the light emitting layer 4519 isradiated to reverse direction of substrate formed TFT on it as shown byan arrow.

Embodiment 6

In this embodiment, the structure of the EL element 4510 which isreversed in a pixel portion shown in FIG. 8 of Embodiment 5 will bedescribed. FIG. 10 will be referred for the explanation. Note that thedifferent point of the structure illustrated in FIG. 8 is only an ELelement portion and a TFT portion, so that the rest will not beexplained.

In FIG. 10, as the switching TFT 4502, a n-channel TFT formed inaccordance with a known method will be used. As the EL driving TFT 4503,a p-channel TFT formed in accordance with a known method is used. It isdesirable to use the switching TFT and the EL driving TFT having thesame polarity.

In Embodiment 6, a transparent conductive film is used as the pixelelectrode (anode) 4525. Specifically, a conductive film made of acompound of indium oxide and zinc oxide is used. Of course, a conductivefilm made of a compound of indium oxide and tin oxide.

Then, a third interlayer insulating film 4526 made of a resin film isformed, a light-emitting layer 4528 is formed. On the light-emittinglayer 4528, an electron injection layer 4529 made of potassium acetylacetonate (acacK), and a cathode 4530 made of an aluminum alloy areformed.

Subsequently, in the same way as in Embodiment 5, a passivation film4532 is formed to prevent oxidation of an organic EL material, therebyforming an EL element 4531.

In the case of an EL element having the structure described inEmbodiment 6, light generated in the light emitting layer 4528 isradiated to the substrate on which TFTs are formed as indicated by anarrow.

Embodiment 7

In order to carry out the driving method of the present invention, thepixel portion needs to have the storage capacitor line 1711 as shown inFIGS. 17A and 17B. The pixel portion structured as such has a largenumber of wirings and hence is inferior in terms of aperture ratio to apixel portion in which one terminal of the storage capacitor 1604 isconnected to the current supply line 1607 as shown in FIGS. 16A and 16B.Accordingly, Embodiment 7 describes a case in which the driving methodof the present invention is carried out with a pixel portion whosewirings are reduced in number by using the gate signal line also as thecurrent supply line. The pixel of this embodiment in which the gatesignal line also serves as the current signal line is disclosed inJapanese Patent Application No. 2000-087683.

Reference is made to FIGS. 11A and 11B. FIGS. 11A and 11B show anexample of the circuit structure for carrying out the driving method ofthe present invention by using the pixel in which the gate signal linealso serves as the current supply line. A pixel portion 1154 is placedin the center of a substrate 1150. A source signal line side drivercircuit 1151 is arranged above the pixel portion 1154. A gate signalline side driver circuit 1152 is put to the left of the pixel portion1154. A storage capacitor line driving circuit 1153 is set to the rightof the pixel portion. FIG. 11B is a circuit diagram showing one pixel.In FIG. 11B, reference symbol 1101 denotes a switching TFT, 1102, an ELdriving TFT, 1103, an EL element, 1104, a storage capacitor, 1105, agate signal line, 1106, a gate signal line one row prior to the gatesignal line 1105, 1107, a source signal line, and 1108, a storagecapacitor line.

The structural feature of this pixel resides in that one of a sourceregion and a drain region of the EL driving TFT 1102 is connected to theprecedent row gate signal line 1106. In FIG. 11B, if the gate signalline 1106 is scanned (k-1)-th and the gate signal line 1105 is scannedk-th, the (k-1)-th row gate signal line 1106 is scanned first and, afterthe scan is completed, scanning of the k-th row gate signal line 1105 isstarted immediately. During the k-th row gate signal line 1105 isscanned, the electric potential of the (k-1)-th row gate signal line1106 that has already been scanned is kept constant. Paying attention tothis fact leads to an arrangement in which supply of current to the ELelement 1103 controlled by the k-th row gate signal line 1105 is made byutilizing the (k-1)-th row gate signal line 1106.

The polarity of the EL driving TFT 1102 may either be of p-channel orn-channel. However, as mentioned before, a p-channel TFT is desirabletaking into consideration proper source grounding and structuralrestrictions on the EL element. The case described in this embodimentuses a p-channel TFT for the EL driving TFT 1102.

Also note that the switching TFT 1101 in this case has to have the samepolarity as the EL driving TFT 1102 from a reason to be revealed later.

The actual driving will be described below. FIGS. 12 and 13 show timingcharts. The illustrated example is of 3 bit gray scale display, and asustain (lights-on) period Ts₃ is shorter than an address (writing)period. The circuit of this embodiment is different from the circuit ofEmbodiment 1 in structure of the pixel portion. However, the samedriving as Embodiment 1 is possible in this embodiment and a clearperiod is provided by increasing the electric potential of the storagecapacitor line 1108 in order to avoid overlap of address (writing)periods. The electric potential of the (k-1)-th row gate signal line1106 is kept constant after the gate signal line 1106 is no longerselected. Until selected next time, the gate signal line 1106 supplies acurrent to the EL element 1103 controlled by the k-th row gate signalline 1105.

The aforementioned reason relating to the polarity of the TFTs will nowbe discussed. According to the foregoing description, the switching TFT1101 and the EL driving TFT 1102 have to have the same polarity. Thismeans that, since a p-channel TFT is used for the EL driving TFT 1102,the switching TFT 1101 also has to be a p-channel TFT in thisembodiment. Assume a case in which the switching TFT 1101 is ann-channel TFT despite the EL driving TFT being a p-channel TFT. In orderto turn the n-channel switching TFT 1101 conductive, a signal inputtedto a gate electrode of the switching TFT 1101 has to be a Hi signal. Inother words, the gate signal lines 1105 and 1106 are given Hi electricpotentials when they are selected whereas they are given LO electricpotentials when they are not selected. With the EL driving TFT 1102being a p-channel TFT, in order to supply a current to the EL element1103, the electric potential has to be higher on the source side of theEL driving TFT 1102 than in an anode 1110 of the EL element. That is,the electric potential of the gate signal line 1106 has to be higher.When the switching TFT 1101 is an n-channel TFT as in the aboveassumption, the electric potential of the gate signal line which is setso as to fit to drive the n-channel TFT turns into a LO electricpotential during the gate signal line is not selected, so that a currentcannot be supplied to the EL element 1103. Therefore, the switching TFThas to be a p-channel TFT if the EL driving TFT 1102 is a p-channel TFT.

According to the circuit structure of this embodiment, a current issupplied, through a connection with the (k-1)-th row gate sig6nal line1106, to the EL element 1103 controlled by the k-th row gate signal line1105. However, a gate signal line other than the line 1106 can providethe same driving as long as it is not selected at that time. Consideringdulling of a signal waveform of the gate signal line, a current issupplied to the EL element desirably by a gate signal two or more rowsahead or behind the line 1105, rather than its adjacent gate signallines. On the other hand, the aperture ratio is decreased as the numberof the connection wirings is increased. Therefore, an operator shouldfind the optimum arrangement taking into consideration the circuitstructure, the characteristics of the TFT elements, etc.

Embodiment 8

The storage capacitor line driving circuit of the present inventionwhich controls the electric potential of the storage capacitor line isan independent circuit in the case shown in Embodiment 1. However, itmay be unified as shown in FIG. 21A. A desirable arrangement for thegate signal line side driver circuit in terms of driving is to place oneon each side of the pixel portion. As shown in FIG. 21B, the gate signalline side driver circuit and the storage capacitor line driving circuitmay be integrated into one circuit to be placed on each side of thepixel portion.

Embodiment 9

In this embodiment, an external light emitting quantum efficiency can beremarkably improved by using an EL material by which phosphorescencefrom a triplet exciton can be employed for emitting a light. As aresult, the power consumption of the EL element can be reduced, thelifetime of the EL element can be elongated and the weight of the ELelement can be lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet exciton (T. Tsutsui, C.Adachi, S. Saito, Photochemical processes in Organized MolecularSystems, ed. K Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an EL material (coumarin pigment) reported bythe above article is represented as follows.

(Chemical formula 1)

-   (M. A. Baldo, D. F. O Brien, Y. You, A. Shoustikov, S. Sibley, M. E.    Thompson, S. R. Forrest, Nature 395 (1998) p. 151)

The molecular formula of an EL material (Pt complex) reported by theabove article is represented as follows.

(Chemical formula 2)

-   (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.    Forrest, Appl. Phys. Lett., 75 (1999) p. 4.)-   (T. Tsutsui, M.-J. Yang, M. Yahiro, K Nakamura, T. Watanabe, T.    Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38    (12B) (1999) L1502)

The molecular formula of an EL material (Ir complex) reported by theabove article is represented as follows.

(Chemical formula 3)

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle. The structureaccording to this embodiment can be freely implemented in combination ofany structures of the first to eighth embodiments.

Embodiment 10

The EL display device, which is an application of driving method ofelectronic device of the present invention, is a self light emittingtype, therefore compared to a liquid crystal display device, it hasexcellent visible properties and is broad in an angle of visibility.Accordingly, the EL display device can be applied to a display portionin various electronic devices. For example, in order to view a TVprogram or the like on a large-sized screen, the EL display device inaccordance with the present invention can be used as a display portionof an EL display having a diagonal size of 30 inches or larger(typically 40 inches or larger).

The EL display includes all kinds of displays to be used for displayinginformation, such as a display for a personal computer, a display forreceiving a TV broadcasting program, a display for advertisementdisplay. Moreover, the driving mathod of electronic equipments inaccordance with the present invention can be used as a display portionof other various electric devices.

As other electronic equipments of the present invention there are: avideo camera; a digital camera; a goggle type display (head mounteddisplay); a navigation system; a sound reproduction device (a car audiostereo, an audio set and so forth); a notebook type personal computer; agame apparatus; a portable information terminal (such as a mobilecomputer, a portable telephone, a portable game machine, or anelectronic book); and an image playback device equipped with a recordingmedium (specifically, device provided with a display portion which playsback images in a recording medium such as a digital versatile diskPlayer (DVD), and displays the images). In particular, in the case ofthe portable information terminal, use of the EL display device ispreferable, since the portable information terminal that is likely to beviewed from a tilted direction is often required to have a wide viewingangle. FIGS. 22A to 23B respectively show various specific examples ofsuch electronic devices.

FIG. 22A shows an EL display containing a casing 3301, a support stand3302, and a display portion 3303. The light emitting device of thepresent invention can be used as the display portion 3303. Such an ELdisplay is a self light emitting type so that a back light is notnecessary. Thus, the display portion can be made thinner than that of aliquid crystal display.

FIG. 22B shows a video camera, and contains a main body 3311, a displayportion 3312, a sound input portion 3313, operation switches 3314, abattery 3315, and an image receiving portion 3316. The electrinic deviceand the driving method of the present invention can be used as thedisplay portion 3312.

FIG. 22C shows a portion (the right-half piece) of an EL display of headmount type, which includes a main body 3321, signal cables 3322, a headmount band 3323, a display portion 3324, an optical system 3325, andisplay device 3326, or the like. The electronic device and the drivingmethod of the present invention is applicable to the display device3326.

FIG. 22D shows an image playback device equipped with a recording medium(specifically, a DVD playback device), and contains a main body 3331, arecording medium (such as a DVD) 3332, operation switches 3333, adisplay portion (a) 3334, and a display portion (b) 3335. The displayportion (a)3334 is mainly used for displaying image information. Thedisplay portion (b) 3335 is mainly used for displaying characterinformation. The light emitting device of the present invention can beused as the display portion (a) 3334 and as the display portion (b)3335. Note that the image playback device equipped with the recordingmedium includes devices such as image playback devices and gamemachines.

FIG. 22E shows a goggle type display device (head mount display device),which includes a main body 3341, a display portion 3342 and an armportion 3343. The electronic device and the driving method of thepresent invention is applicable to the display portion 3342.

FIG. 22F is a personal computer, and contains a main body 3351, a casing3352, a display portion 3353, and a keyboard 3354. The electronic deviceand the driving method of the present invention is applicable to thedisplay portion 3353.

Note that if the luminance of EL material increases in the future, thenit will become possible to use the light emitting device of the presentinvention in a front type or a rear type projector by expanding andprojecting light containing output image information with a lens or thelike.

Further, the above electric devices display often informationtransmitted through an electronic communication circuit such as theInternet and CATV (cable television), and particularly situations ofdisplaying moving images is increasing. The response speed of ELmaterials is so high that the EL devices are good for display of movingimage.

In addition, since the light emitting device conserves power in thelight emitting portion, it is preferable to display information so as tomake the light emitting portion as small as possible. Consequently, whenusing the EL display device in a display portion mainly for characterinformation, such as in a portable information terminal, in particular aportable telephone or a sound reproduction device, it is preferable todrive the light emitting device so as to form character information bythe light emitting portions while non-light emitting portions are set asbackground.

FIG. 23A shows a portable telephone, and contains a main body 3401, asound output portion 3402, a sound input portion 3403, a display portion3404, operation switches 3405, and an antenna 3406. The light emittingdevice of the present invention can be used as the display portion 3404.Note that by displaying white color characters in a black colorbackground, the display portion 3404 can suppress the power consumptionof the portable telephone.

FIG. 23B shows a sound reproduction device, a car audio equipment in aconcrete term, and contains a main body 3411, a display portion 3412,and operation switches 3413 and 3414. The light emitting device of thepresent invention can be used as the display portion 3412. Further, acar mounting audio stereo is shown in this embodiment, but a fixed typeaudio playback device may also be used. Note that, by displaying whitecolor characters in a black color background, the display portion 3414can suppress the power consumption.

As described above, the application range of this invention is extremelywide, and it may be used for electric devices in various fields.Further, the electric device of this embodiment may be obtained by usinga light emitting device freely combining the structures of the first toninth embodiments.

Effects of the present invention will be listed. Firstly, according tothe present invention, pixels of another row can be brought intonon-display state even when signals are inputted to pixels of one row.This makes it possible to set the length of a sustain (lights-on) periodwithout restriction, even to a shorter length than an address (writing)period in pixels of the respective rows, thereby providing multi-grayscale.

Secondly, in the driving method of the present invention, the EL elementis brought into non-display state by changing the electric potential ofthe storage capacitor line. A constant electric potential is thus givento a cathode wiring. Since it is not a pulsated signal as in prior art,various problems caused by dulled voltage waveform of the cathode wiringcan be avoided.

Lastly, there is no need to newly add a transistor, a capacitor, or awiring to the components of the pixel portion. Therefore the quality ofdisplayed image can be improved without lowering the aperture ratio.

1-27. (canceled)
 28. An electronic device comprising: a first transistorhaving a first gate electrode electrically connected to a gate signalline and first impurity regions, one of the first impurity regionselectrically connected to a source signal line; a capacitor storagehaving electrodes, one of the electrodes electrically connected to acapacitor storage line; a second transistor having a second gateelectrode electrically connected to the capacitor storage and anotherone of first impurity regions, and second impurity regions, one of thesecond impurity regions electrically connected to a current supply line;an EL element having electrodes, one of electrodes electricallyconnected to another one of the second impurity regions, wherein anelectrical potential of another one of the electrodes of the EL elementis higher than that of the current supply line.
 29. An electronic devicecomprising: a first transistor having a first gate electrodeelectrically connected to a gate signal line and first impurity regions,one of the first impurity regions electrically connected to a sourcesignal line; a capacitor storage having electrodes, one of theelectrodes electrically connected to a capacitor storage line; a secondtransistor having a second gate electrode electrically connected to thecapacitor storage and another one of the first impurity regions, andsecond impurity regions, one of the second impurity regions electricallyconnected to a current supply line; an EL element having electrodes, oneof the electrodes electrically connected to another one of the secondimpurity regions, wherein an electric current flows from another one ofthe electrodes of the EL element to the current supply line through theEL element.
 30. An electronic device comprising: a first transistorhaving a first gate electrode electrically connected to a gate signalline and first impurity regions, one of the first impurity regionselectrically connected to a source signal line; a capacitor storagehaving electrodes, one of the electrodes electrically connected to acapacitor storage line; a capacitor storage line driving circuitelectrically connected to the capacitor storage line; a secondtransistor having a second gate electrode electrically connected to thecapacitor storage and another one of first impurity regions, and secondimpurity regions, one of the second impurity regions electricallyconnected to a current supply line; an EL element having electrodes, oneof electrodes electrically connected to another one of the secondimpurity regions, wherein an electrical potential of another one of theelectrodes of the EL element is higher than that of the current supplyline.
 31. An electronic device comprising: a first transistor having afirst gate electrode electrically connected to a gate signal line andfirst impurity regions, one of the first impurity regions electricallyconnected to a source signal line; a capacitor storage havingelectrodes, one of the electrodes electrically connected to a capacitorstorage line; a capacitor storage line driving circuit electricallyconnected to the capacitor storage line; a second transistor having asecond gate electrode electrically connected to the capacitor storageand another one of the first impurity regions, and second impurityregions, one of the second impurity regions electrically connected to acurrent supply line; an EL element having electrodes, one of theelectrodes electrically connected to another one of the second impurityregions, wherein an electric current flows from another one of theelectrodes of the EL element to the current supply line through the ELelement.
 32. An electronic device according to claim 28, wherein anelectric potential of the capacitor storage line is varied by a signalinputted from a capacitor signal driving circuit.
 33. An electronicdevice according to claim 29, wherein an electric potential of thecapacitor storage line is varied by a signal inputted from a capacitorsignal driving circuit.
 34. An electronic device according to claim 30,wherein an electric potential of the capacitor storage line is varied bya signal inputted from the capacitor signal driving circuit.
 35. Anelectronic device according to claim 31, wherein an electric potentialof the capacitor storage line is varied by a signal inputted from thecapacitor signal driving circuit.
 36. An electronic device according toclaim 28, wherein the electronic device is a device selected from thegroup consisting of an EL display, a video camera, a head-mount display,a DVD player, a personal computer, a cellular phone and an audio systemfor automobiles.
 37. An electronic device according to claim 29, whereinthe electronic device is a device selected from the group consisting ofan EL display, a video camera, a head-mount display, a DVD player, apersonal computer, a cellular phone and an audio system for automobiles.38. An electronic device according to claim 30, wherein the electronicdevice is a device selected from the group consisting of an EL display,a video camera, a head-mount display, a DVD player, a personal computer,a cellular phone and an audio system for automobiles.
 39. An electronicdevice according to claim 31, wherein the electronic device is a deviceselected from the group consisting of an EL display, a video camera, ahead-mount display, a DVD player, a personal computer, a cellular phoneand an audio system for automobiles.
 40. A method of driving anelectronic device, with one frame period comprising n sub-frame periodsSF₁, SF₂, . . . , SF_(n), the n sub-frame periods each comprisingaddress periods Ta₁, Ta₂, . . . , Ta_(n) and sustain periods Ts₁, Ts₂, .. . , Ts_(n), comprising the steps of: inputting a first signal to apixel comprising a light emitting element from a source signal lineduring each address period, wherein a capacitor storage line ismaintained at a first potential; turning on the light emitting elementduring each sustain period, wherein the capacitor storage line ismaintained at the first potential; providing a clear period Tc_(m)during a period from an end of the sustain period Ts_(m)(1≦m≦n−1) of asub-frame period SF_(m) through until a start of the address periodTa_(m+1) of a sub-frame period SF_(m+1), wherein the capacitor storageline is maintained at a second potential.
 41. A method of driving anelectronic device, with one frame period comprising n sub-frame periodsSF₁, SF₂, . . . , SF_(n), the n sub-frame periods each comprisingaddress periods Ta₁, Ta₂, . . . , Ta_(n) and sustain periods Ts₁, Ts₂, .. . , Ts_(n), comprising the steps of: inputting a first signal to apixel comprising a light emitting element from a source signal lineduring an address period Ta_(n) of a j-th (0<j) frame sub-frame periodSF_(n), wherein a capacitor storage line is maintained at a firstpotential; turning on the light emitting element during a sustain periodTs_(n) of a j-th (0<j) frame sub-frame period SF_(n), wherein acapacitor storage line is maintained at a first potential; providing aclear period Tc_(n) during a period from an end of the sustain periodTs_(n) through until a start of the address period Ta₁ of a (j+1)-thperiod frame sub-frame period SF₁, wherein the capacitor storage line ismaintained at a second potential.
 42. A method of driving an electronicdevice, with one frame period comprising n sub-frame periods SF₁, SF₂, .. . , SF_(n), the n sub-frame periods each comprising address periodsTa₁, Ta₂, . . . , Ta_(n) and sustain periods Ts₁, Ts₂, . . . , Ts_(n),providing a clear period Tc_(k) during a period from an end of thesustain period TS_(k)(1≦m≦n) of a sub-frame period SF_(k) through untila start of the address period Ta_(k+1) of a sub-frame period SF_(k+1),wherein the capacitor storage line is maintained at a second potential.43. A method of driving an electronic device as claimed in claim 40,wherein a clear signal inputted during the clear period is provided byincreasing or lowering the electric potential of a capacitor storageline by means of a signal inputted from a capacitor storage line drivingcircuit.
 44. A method of driving an electronic device as claimed inclaim 41, wherein a clear signal inputted during the clear period isprovided by increasing or lowering the electric potential of a capacitorstorage line by means of a signal inputted from a capacitor storage linedriving circuit.
 45. A method of driving an electronic device as claimedin claim 42, wherein a clear signal inputted during the clear period isprovided by increasing or lowering the electric potential of a capacitorstorage line by means of a signal inputted from a capacitor storage linedriving circuit.
 46. A method of driving an electronic device as claimedin claim 40, wherein an EL element does not emit light during the clearperiod irrespective of an image signal.
 47. A method of driving anelectronic device as claimed in claim 41, wherein an EL element does notemit light during the clear period irrespective of an image signal. 48.A method of driving an electronic device as claimed in claim 42, whereinan EL element does not emit light during the clear period irrespectiveof an image signal.
 49. An electronic device operated by a drivingmethod in which: one frame period comprises n sub-frame periods SF₁,SF₂, . . . , SF_(n); the n sub-frame periods each comprises addressperiods Ta₁, Ta₂, . . . , Ta_(n) and sustain periods Ts₁, Ts₂, . . . ,Ts_(n); inputting a first signal to a pixel comprising a light emittingelement from a source signal line during each address period, wherein acapacitor storage line is maintained at a first potential; turning onthe light emitting element during each sustain period, wherein thecapacitor storage line is maintained at the first potential; providing aclear period Tc_(m) during a period from an end of the sustain periodTs_(m)(1≦m≦n−1) of a sub-frame period SF_(m) through until a start ofthe address period Ta_(m+1) of a sub-frame period SF_(m+1), wherein thecapacitor storage line is maintained at a second potential.
 50. Anelectronic device operated by a driving method in which: one frameperiod comprises n sub-frame periods SF₁, SF₂, . . . , SF_(n); the nsub-frame periods each comprises address periods Ta₁, Ta₂, . . . ,Ta_(n) and sustain (lights-on) periods Ts₁, Ts₂, . . . , Ts_(n);inputting a first signal to a pixel comprising a light emitting elementfrom a source signal line during an address period Ta_(n) of a j-th(0<j) frame sub-frame period SF_(n), wherein a capacitor storage line ismaintained at a first potential; turning on the light emitting elementduring a sustain period Ts_(n) of a j-th (0<j) frame sub-frame periodSF_(n), wherein a capacitor storage line is maintained at a firstpotential; providing a clear period Tc_(n) during a period from an endof the sustain period Ts_(n) through until a start of the address periodTa₁ of a (j+1)-th period frame sub-frame period SF₁, wherein thecapacitor storage line is maintained at a second potential.
 51. Anelectronic device wherein: one frame period comprises n sub-frameperiods SF₁, SF₂, . . . , SF_(n); the n sub-frame periods each comprisesaddress periods Ta₁, Ta₂, . . . , Ta_(n) and sustain periods Ts₁, Ts₂, .. . , Ts_(n); and, providing a clear period Tc_(k) during a period froman end of the sustain period Ts_(k)(1≦k≦n) of a sub-frame period SF_(k)through until a start of the address period Ta_(k+1) of a sub-frameperiod SF_(k+1), wherein the capacitor storage line is maintained at asecond potential.
 52. An electronic device as claimed in claim 49,wherein a clear signal inputted during the clear period is provided byincreasing or lowering the electric potential of a capacitor storageline by means of a signal inputted from a capacitor storage line drivingcircuit.
 53. An electronic device as claimed in claim 50, wherein aclear signal inputted during the clear period is provided by increasingor lowering the electric potential of a capacitor storage line by meansof a signal inputted from a capacitor storage line driving circuit. 54.An electronic device as claimed in claim 51, wherein a clear signalinputted during the clear period is provided by increasing or loweringthe electric potential of a capacitor storage line by means of a signalinputted from a capacitor storage line driving circuit.
 55. Anelectronic device as claimed in claim 49, wherein an EL element does notemit light during the clear period irrespective of an image signal. 56.An electronic device as claimed in claim 50, wherein an EL element doesnot emit light during the clear period irrespective of an image signal.57. An electronic device as claimed in claim 51, wherein an EL elementdoes not emit light during the clear period irrespective of an imagesignal.
 58. A method of driving a electronic device according to claim49, wherein said electronic device is a device selected from the groupconsisting of: an EL display, a video camera, a head-mount display, aDVD player, a personal computer, a cellular phone and an audio systemfor automobiles.
 59. A method of driving a electronic device accordingto claim 50, wherein said electronic device is a device selected fromthe group consisting of: an EL display, a video camera, a head-mountdisplay, a DVD player, a personal computer, a cellular phone and anaudio system for automobiles.
 60. A method of driving a electronicdevice according to claim 51, wherein said electronic device is a deviceselected from the group consisting of: an EL display, a video camera, ahead-mount display, a DVD player, a personal computer, a cellular phoneand an audio system for automobiles.
 61. An electronic device accordingto claim 49, wherein said electronic device is a device selected fromthe group consisting of: an EL display, a video camera, a head-mountdisplay, a DVD player, a personal computer, a cellular phone and anaudio system for automobiles.
 62. An electronic device according toclaim 50, wherein said electronic device is a device selected from thegroup consisting of: an EL display, a video camera, a head-mountdisplay, a DVD player, a personal computer, a cellular phone and anaudio system for automobiles.
 63. An electronic device according toclaim 51, wherein said electronic device is a device selected from thegroup consisting of: an EL display, a video camera, a head-mountdisplay, a DVD player, a personal computer, a cellular phone and anaudio system for automobiles.